Variable-gain differential input and output amplifier

ABSTRACT

A variable-gain amplifier with a differential input and differential output, including an attenuator block, receiving an input voltage and providing, on several outputs, voltages, each of which is equal to the attenuated input voltage; differential transconductor elements, each having a first input connected to a respective output of the attenuator block, and generating first and second positive currents and first and second negative currents; a current source assembly adapted to controlling the transconductance of each differential transconductor element according to an analog control signal; and an output block converting first and second input currents into a differential output voltage and providing a second input of each differential transconductor element with a feedback voltage depending on the output voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable-gain amplifier, and inparticular to such an amplifier with differential input and output, thegain of which varies exponentially as a response to an analog controlvoltage, and having a small harmonic distortion and reduced noise.

2. Description of the Related Art

FIG. 1 schematically shows an amplifier, which can be made in integratedcircuit form, having a gain that varies exponentially as a response toan analog control voltage Vcom. The amplifier is intended for receivinga differential voltage V1in-V2in and for providing as a response adifferential voltage V1out-V2out.

Voltage V1in is provided to an attenuator network AT1 referenced to aground potential GND and having n output nodes O1 i (i ranging between 1and n). Each output node O1 i generates a control voltage equal to inputvoltage V1in attenuated according to a predetermined ratio, for example2^(i) when network AT1 is an R/2R network. The amplifier includes afirst assembly of transconductor elements G1 i controllable by a currentand formed, for example, of two bipolar transistors connected as shown.Each transconductor element G1 i receives on a first input the voltagegenerated by the node O1 i of same rank i. Each transconductor elementG1 i is provided for providing a positive current I1 ⁺ on a currentoutput terminal 2 and a negative current I1 ⁻ on a current outputterminal 4. Output terminals 2 and 4 respectively form the first andsecond input terminals of an output block 6. Output block 6 includes acurrent-to-voltage conversion element 8 having two input terminals andone output terminal. The two input terminals of converter 8 correspondto the two input terminals of block 6. Two current sources CS1, CS2 arearranged between a supply voltage VDD and respectively the first andsecond input terminals of block 6. The output terminal of converter 8 isconnected to potential GND via a dividing bridge formed of two resistorsR1, R2, the junction point of which is connected to provide a feedbacksignal to a second input of each transconductor element G1 i. The outputterminal of converter 8 provides a voltage V1out. The amplifier furtherincludes an assembly of controllable current sources 10 having n outputterminals S1 i. Each output terminal S1 i is connected to control thetransconductance of the transconductor element G1 i of same rank i. Theassembly of current sources 10 is controlled by an analog controlvoltage Vcom.

Voltage V2in is provided to an attenuator network AT2 identical tonetwork AT1, referenced to potential GND, and having n output nodes O2i. The amplifier includes a second assembly of transconductor elementsG2 i controllable by a current, each of which receives on a first inputthe voltage generated by the node O2 i of same rank i. Eachtransconductor element G2 i is provided to provide a positive current I2⁺ on a current output terminal 12 and a negative current I2 ⁻ on acurrent output terminal 14, respectively forming the first and secondinput terminals of an output block 16. Output block 16 includes acurrent-to-voltage conversion element 18, having its two input terminalsconnected to the two input terminals of block 16. Two current sourcesCS3, CS4 are respectively arranged between supply voltage VDD and thefirst and second input terminals of block 16. The output of converter 18is connected to potential GND via a dividing bridge formed of tworesistors R3, R4, the junction point of which is connected to provide afeedback signal to a second input of each transconductor element G2 i.The output terminal of converter 18 provides a voltage V2out. Assembly10 of current sources includes n output terminals S2 i, each of which isconnected to a transconductor element of same rank i.

Attenuator networks AT1 and AT2 form a differential attenuator blockreceiving differential input signal V1in-V2in. Each pair of output nodesO1 i, O2 i of the differential attenuator block provides a controlvoltage to the pair of transconductor elements G1 i, G2 i of same rank.Each pair of transconductor elements G1 i, G2 i forms a differentialtransconductor element. Output blocks 6 and 16 form a differentialoutput block receiving the currents provided by each differentialtransconductor element G1 i, G2 i and generating the differential outputsignal V1out-V2out. The output pairs S1 i, S2 i of current sourceassembly 10 generate matched pairs of control currents to control eachdifferential transconductor element G1 i, G2 i. The amplifier describedhereabove is the object of a still unpublished French patent applicationand does not belong to the state of the art.

For such an amplifier to have a satisfactory operation,current-to-voltage conversion elements 8 and 18 must be matched. If not,the amplifier half which receives voltage V1in and generates voltageV1out and the amplifier half which receives voltage V2in and generatesvoltage V2out have different gains and bandwidths, which causes adistortion of the amplifier output signal. In practice, it is difficultto form two matched current-to-voltage converters 8 and 18.

Further, current source pairs CS1, CS2, and CS3, CS4 operate in adecorrelated manner, which can contribute to increasing the noise levelof the amplifier.

Further, each of voltage signals V1out and V2out is generated by anon-symmetrical amplifier which does not suppress the distortion due tothe harmonic of second order.

BRIEF SUMMARY OF THE INVENTION

The disclosed embodiments of the present invention provide avariable-gain amplifier with a differential input and output, the gainof which varies exponentially as a response to an analog controlvoltage, and which exhibits a small harmonic distortion and low noise.

To achieve the foregoing, an embodiment of the present inventionespecially provides a variable-gain amplifier with differential inputand output, including an attenuator block referenced to the common modevoltage of the amplifier output, receiving an input voltage and adaptedto providing, on several outputs, voltages, each of which is equal tothe input voltage attenuated according to a predetermined ratio;differential transconductor elements controllable by a current, eachdifferential transconductor element having a first input connected to anoutput of the attenuator block, each differential transconductor elementgenerating first and second positive currents and first and secondnegative currents; a current source assembly adapted to controlling thetransconductance of each differential transconductor element accordingto an analog control signal; and an output block converting first andsecond input currents into a differential output voltage and providing asecond input of each differential transconductor element with a feedbackvoltage depending on the output voltage, the first input current beingequal to the sum of the first positive currents and of the secondnegative currents and the second input current being equal to the sum ofthe second positive currents and of the first negative currents.

According to an embodiment of the present invention, the attenuatorblock includes two attenuator networks of R/2R type respectivelyreceiving first and second input voltages, each output of the attenuatorblock including an output node of each attenuator network, the outputnode generating an attenuated voltage equal to the input voltagereceived by said network, attenuated according to the predeterminedratio specific to said output; each differential transconductor elementincludes two pairs of bipolar transistors, the emitters of thetransistors of a first pair being connected to a first current controlterminal of the differential transconductor element, and the emitters ofthe transistors of the second pair being connected to a second currentcontrol terminal of the differential transconductor element, the firstinput of the differential transconductor element being formed by thebases of the first two transistors of the two transistor pairs, thesecond input of the differential transconductor element being formed bythe bases of the two second transistors of the two transistor pairs, thetwo bases forming the first input of the differential transconductorelement being respectively connected to the output nodes of the outputof the attenuator block connected to the differential transconductorelement, the two bases forming the second input of the differentialtransconductor element being submitted to the feedback voltage, thecollectors of the first transistors generating the first and secondpositive currents, and the collectors of the second transistorsgenerating the first and second negative currents; and the currentsource assembly includes pairs of control terminals connected to thefirst and second current control terminals of each differentialtransconductor element.

According to an embodiment of the present invention, the output blockincludes: a current-to-voltage conversion element having first andsecond input terminals respectively provided for receiving the first andsecond input currents, generating the common mode voltage on a commonmode output terminal, and respectively generating first and secondoutput voltages on first and second output terminals, first and secondresistive dividing bridges respectively arranged between the first andsecond output terminals of the current-to-voltage conversion element andthe common mode voltage, the midpoint of the first and second resistivedividing bridges generating the feedback voltage; and first and secondcurrent sources respectively arranged between a supply voltage and thefirst and second input terminals of the output block.

According to an embodiment of the present invention, thecurrent-to-voltage conversion element includes: first and secondidentical P-type MOS transistors, having their sources connected to thesupply voltage, having their gates connected to each other, having theirrespective drains connected to the first and second input terminals ofthe current-to-voltage conversion element; third and fourth identicalP-type MOS transistors, having their sources connected to the supplyvoltage, having their respective gates connected to the respectivedrains of the first and second MOS transistors, having their respectivedrains connected to the first and second output terminals of thecurrent-to-voltage conversion element; third and fourth current sourcesarranged between the respective drains of the third and fourthtransistors and a first reference voltage; two identical resistors eachconnected between the respective drains of the third and fourthtransistors and the common mode output terminal; and a differentialamplifier having a first input connected to the common mode outputterminal, having a second input connected to a second reference voltage,and having its output connected to the gates of the first and secondtransistors.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing objects, features and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments, in conjunction with the accompanying drawings,wherein:

FIG. 1, previously described, schematically shows a conventionalvariable-gain amplifier;

FIG. 2 schematically shows a variable-gain amplifier according to thepresent invention;

FIG. 3 schematically shows an embodiment of the current-to-voltageconversion element of FIG. 2; and

FIG. 4 schematically shows an embodiment of the controllable currentsource assembly of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Only those elements necessary to the understanding of the presentinvention have been shown in the following drawings. Same referencenumbers represent the same elements in the previous drawing and in thefollowing drawings.

FIG. 2 schematically shows an embodiment of a variable-gain amplifieraccording to the present invention, intended for receiving adifferential voltage V1in-V2in and for providing as a response adifferential voltage V1out-V2out. The amplifier includes a firstattenuator network AT1 of R/2R type receiving voltage V1in on an inputterminal and having n output nodes O1 i. According to the presentinvention, attenuator network AT1 is referenced to common mode voltageVCM of the amplifier output. The amplifier includes a first assembly oftransconductor elements G1 i controllable by a current, each of whichreceives on a first input the voltage generated by the node O1 i of samerank i. Each transconductor element G1 i includes a pair of bipolartransistors, the emitters of which are connected to a current controlterminal of element G1 i. The base of a first transistor forms a firstinput of element G1 i. The base of the second transistor forms a secondinput of element G1 i. The collector of the first transistor provides apositive current I1 ⁺ to a current output terminal 2. The collector ofthe second transistor provides a negative current I1 ⁻ to a currentoutput terminal 4. The amplifier includes a second attenuator networkAT2 identical to network AT1, receiving voltage V2in on an inputterminal, having n output nodes O2 i, and referenced to voltage VCM. Theamplifier includes a second assembly of transconductor elements G2 icontrollable by a current, each of which receives on a first input thevoltage generated by the node O2 i of same rank i. Each transconductorelement G2 i includes a pair of bipolar transistors having theiremitters connected to a current control terminal of element G2 i. Thebase of a first transistor forms a first input of element G2 i. The baseof the second transistor forms a second input of element G2 i. Thecollector of the first transistor provides a positive current I2 ⁺ to acurrent output terminal 12. The collector of the second transistorprovides a negative current I2 ⁻ to a current output terminal 14.

The transconductor elements G1 i and G2 i of same rank form adifferential transconductor element G1 i, G2 i. The transconductance ofeach differential transconductor element G1 i, G2 i is controlled by theoutput pairs S1 i, S2 i of a controllable current assembly 10 connectedto the current control terminals of the differential transconductorelement. An example of forming of current source assembly 10 isdescribed hereafter.

According to this embodiment of the present invention, the amplifierincludes a single output block 26 having first and second inputterminals IN1, IN2 and including a current-to-voltage conversion element28. Current-to-voltage conversion element 28 has first and second inputterminals IN1′ and IN2′ respectively connected to terminals IN1 and IN2.Current-to-voltage conversion element 28 further includes first andsecond output terminals OUT1, OUT2 and a common mode output terminalgenerating voltage VCM. Terminal IN1 is connected to current outputterminals 2 and 14. Terminal IN1 receives a first input current equal tothe sum of the positive currents I1 ⁺ generated by transconductorelements G1 i and of the negative currents I2 ⁻ generated bytransconductor elements G2 i. Terminal IN2 is connected to currentoutput terminals 4 and 12. Terminal IN2 receives a second input currentequal to the sum of the negative currents I1 ⁻ generated bytransconductor elements G1 i and of the positive currents I2 ⁺ generatedby transconductor elements G2 i. A current source CS5 is arrangedbetween terminal IN1 and a supply voltage VDD. A current source CS6 isarranged between terminal IN2 and supply voltage VDD. The first outputterminal OUT1 of element 28 is connected to voltage VCM via a firstdividing bridge formed of two resistors R1, R2. The midpoint of thefirst dividing bridge is connected to provide a feedback signal to thesecond input terminal of each transconductor element G1 i. The secondoutput terminal OUT2 of element 28 is connected to voltage VCM via asecond dividing bridge formed of two resistors R3, R4. The midpoint ofthe second dividing bridge is connected to provide a feedback signal tothe second input terminal of each transconductor element G2 i. Outputterminal OUT1 provides output voltage V1 out and output terminal OUT2provides output voltage V2out.

The amplifier generates differential voltage signal V1out-V2out fromdifferential voltage signal V1in-V2in by means of a singlecurrent-to-voltage conversion element 28, which enables suppressingdistortion problems due to the difficulties of matching the twocurrent-to-voltage conversion elements of a conventional amplifier.

The first and second input terminals IN1 and IN2 of current-to-voltageconversion element 28 are each connected to a single current source,respectively CS5 and CS6. The two current sources CS5 and CS6 replacethe two current source pairs CS1, CS2 and CS3, CS4 operating in adecorrelated way of a conventional amplifier, which enables reducing thenoise coming from power supply VDD.

The amplifier according to this embodiment of the present invention is asymmetrical assembly that eliminates the second harmonic term fromoutput signal V1out-V2out.

The input of the amplifier is a differential signal referenced to commonmode voltage VCM. As a result, for a given dynamic range, a lower supplyvoltage than in the case of two input terminals referenced to ground canbe used.

FIG. 3 schematically shows an embodiment of a current-to-voltageconversion element 28. Two identical P-type MOS transistors T1 and T2have their sources connected to voltage VDD and their gates connected toeach other. The respective drains of transistors T1 and T2 are connectedto terminals IN1′ and IN2′. Two identical P-type MOS transistors T3 andT4 have their sources connected to voltage VDD and their respectivegates connected to terminals IN1′ and IN2′. The respective drains oftransistors T3 and T4 are connected to terminals OUT1 and OUT2. Currentsources CS7 and CS8 are respectively arranged between the respectivedrains of transistors T3 and T4 and a ground voltage GND. The drain ofeach of transistors T3 and T4 is connected to the common mode outputterminal by a resistor R. A differential amplifier 30 has a first inputconnected to the common mode output voltage and a second input connectedto a reference voltage Vref. The output of amplifier 30 is connected tothe gates of transistors T1 and T2.

FIG. 4 schematically shows an embodiment of controllable current sourceassembly 10. Current source assembly 10 includes n pairs of outputterminals S1 i, S2 i and two control terminals A and B. Control voltageVcom is provided across terminals A and B. Each output terminal S1 i, S2i is respectively connected to the drain of N-channel MOS transistors T1i, T2 i. Transistors T1 i and T2 i are matched. The source oftransistors T1 i, T2 i is connected to ground (GND). Transistor pair T1i, T2 i is associated with an N-channel MOS transistor T3 i and with aP-channel MOS transistor T4 i. The source of transistor T3 i isgrounded. The gate and drain of transistor T3 i are interconnected. Thegate of transistors T1 i, T2 i is connected to the gate of transistor T3i so that two matched currents depending on the current in transistor T3i run through transistors T1 i and T2 i. The drain of transistor T3 i isconnected to the drain of transistor T4 i, so that these transistors arein series. The source of transistor T4 i is connected to supply voltageVDD via a single constant current source CS9. The gate of transistor T4i is connected to a node Ni of a control means 32 formed of resistors R′and of current sources CS′. In control means 32, the first node N1 iscoupled to terminal A, the last node Nn is coupled to terminal B, eachnode Nj (j ranging between 1 and n−1) is connected to node Nj+1 via aresistor R′ and each node Nj (j ranging from 2 to n−1) is connected to afirst terminal of a constant current source CS′ specific to this node.Control means 32 is provided to successively progressively turn on, thenprogressively turn off, each transistor T4 i when voltage Vcom undergoesa predetermined variation, so that the sum of the currents flowingthrough transistors T4 i is substantially constant. Each output terminalS1 i, S2 i provides a current depending on the current flowing throughtransistor T4 i.

The disclosed embodiments of the present invention may have variousalterations, modifications, and improvements that will readily occur tothose skilled in the art. The present invention has been described inrelation with a particular controllable current source assembly 10, butthose skilled in the art may adapt the present invention to a currentsource assembly having an equivalent function. For example, such anassembly may be obtained by doubling the current source assembly used tocontrol the variable-gain amplifier with a non-differential input andoutput described in U.S. Pat. No. 5,077,541.

The present invention has been described in relation with a differentialattenuator block using R/2R dividing networks, but those skilled in theart may adapt the same to other attenuator blocks having differentattenuation ratios.

The present invention has also been described in relation with aspecific current-to-voltage conversion element 28, but those skilled inthe art may adapt the present invention to any equivalentcurrent-to-voltage conversion element.

In the foregoing description, first input IN1 of block 26 receives thesum of the positive currents I1 ⁺ generated by transconductor elementsG1 i and of the negative currents I2 ⁻ generated by transconductorelements G2 i. Similarly, the second input IN2 of block 26 receives thesum of the negative currents I1 ⁻ generated by transconductor elementsG1 i and of the positive currents I2 ⁻ generated by transconductorelements G2 i. However, those skilled in the art may adapt the presentinvention to the case where the first input IN1 of block 26 receives thesum of the negative currents I1 ⁻ generated by the transconductorelements G1 i and of the positive currents I2 ⁺ generated by thetransconductor elements G2 i, and where the second input IN2 of block 26receives the sum of the positive currents I1 ⁺ generated by thetransconductor elements G1 i and of the negative currents I2 ⁻ generatedby the transconductor elements G2 i.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereof.

1. A variable-gain amplifier with differential input and output,comprising: an attenuator block referenced to a common mode voltage ofan output of the amplifier, receiving an input voltage and adapted toproviding, on several outputs, voltages, each of which is equal to theinput voltage attenuated according to a predetermined ratio,differential transconductor elements controllable by a current, eachdifferential transconductor element having a first input connected to anoutput of the attenuator block, each differential transconductor elementgenerating first and second positive currents and first and secondnegative currents, a current source assembly adapted to controlling thetransconductance of each differential transconductor element accordingto an analog control signal, and an output block converting first andsecond input currents into a differential output voltage and providing asecond input of each differential transconductor element with a feedbackvoltage depending on the output voltage, the first input current beingequal to the sum of the first positive currents and of the secondnegative currents and the second input current being equal to the sum ofthe second positive currents and of the first negative currents.
 2. Theamplifier of claim 1, wherein: the attenuator block comprises twoattenuator networks of R/2R type, respectively receiving first andsecond input voltages, each output of the attenuator block comprising anoutput node of each attenuator network, the output node generating anattenuated voltage equal to the input voltage received by said network,attenuated according to the predetermined ratio specific to said output,each differential transconductor element comprising two pairs of bipolartransistors, the emitters of the transistors of a first pair beingconnected to a first current control terminal of the differentialtransconductor element, and the emitters of the transistors of thesecond pair being connected to a second current control terminal of thedifferential transconductor element, the first input of the differentialtransconductor element being formed by the bases of the first twotransistors of the two transistor pairs, the second input of thedifferential transconductor element being formed by the bases of the twosecond transistors of the two transistor pairs, the two bases formingthe first input of the differential transconductor element beingrespectively connected to the output nodes of the output of theattenuator block connected to the differential transconductor element,the two bases forming the second input of the differentialtransconductor element being submitted to the feedback voltage, thecollectors of the first transistors generating the first and secondpositive currents, and the collectors of the second transistorsgenerating the first and second negative currents, and the currentsource assembly includes pairs of control terminals connected to thefirst and second current control terminals of each differentialtransconductor element.
 3. The amplifier of claim 1, wherein the outputblock comprises: a current-to-voltage conversion element having firstand second input terminals respectively provided for receiving the firstand second input currents, generating the common mode voltage on acommon mode output terminal, and respectively generating first andsecond output voltages on first and second output terminals, first andsecond resistive dividing bridges respectively coupled between the firstand second output terminals of the current-to-voltage conversion elementand the common mode voltage, the midpoint of the first and secondresistive dividing bridges generating the feedback voltage, and firstand second current sources respectively coupled between a supply voltageand the first and second input terminals of the output block.
 4. Theamplifier of claim 3, wherein the current-to-voltage conversion elementcomprises: first and second identical P-type MOS transistors, havingtheir sources connected to the supply voltage, having their gatesconnected to each other, having their respective drains connected to thefirst and second input terminals of the current-to-voltage conversionelement, third and fourth identical P-type MOS transistors, having theirsources connected to the supply voltage, having their respective gatesconnected to the respective drains of the first and second MOStransistors, having their respective drains connected to the first andsecond output terminals of the current-to-voltage conversion element,third and fourth current sources arranged between the respective drainsof the third and fourth transistors and a first reference voltage, twoidentical resistors each connected between the respective drains of thethird and fourth transistors and the common mode output terminal, and adifferential amplifier having a first input connected to the common modeoutput terminal, having a second input connected to a second referencevoltage, and having its output connected to the gates of the first andsecond transistors.
 5. A variable gain amplifier having a differentialinput and differential output, the amplifier comprising: an attenuatorcircuit configured to receive an input voltage from the differentialinput, to attenuate the input voltage, and to provide a plurality ofattenuated output voltages; a plurality of differential transconductorcircuits, each differential transconductor circuit having a first inputconnected to a respective attenuated output voltage from the attenuatorcircuit and configured to generate first and second positive currentsand first and second negative currents; a current source assemblyconfigured to control a transconductance of each differentialtransconductor circuit according to an analog control signal; and anoutput block configured to convert the first and second input currentsinto a differential output voltage and to provide a second input of eachdifferential transconductor circuit with a feedback voltage that isdependent on the differential output voltage.
 6. A variable-gainamplifier with differential input and output, the amplifier comprising:a differential input stage configured to receive first and seconddifferential inputs and to generate first and second positive currentsand first and second negative currents; and an output stage configuredto receive and convert the first and second negative currents and thefirst and second positive currents into first and second differentialoutput voltages, and to provide a feedback voltage to the differentialinput stage dependent on an output of the differential output voltages.7. A variable-gain amplifier with differential input and output,comprising: a differential input stage configured to receive first andsecond differential inputs and to generate first and second positivecurrents and first and second negative currents; and an output stageconfigured to receive and convert the first and second positive currentsand the first and second negative currents into first and seconddifferential output voltages on first and second outputs and to providea feedback voltage to the differential input stage that is dependent onthe output voltages on the first and second outputs, the output stagefurther comprising a first resistive bridge comprising first and secondresistors coupled in series with each other and between the first outputand a common mode terminal, and a second resistive bridge comprisingthird and fourth resistors coupled in series with each other and betweenthe second output and the common mode terminal.
 8. A variable-gainamplifier with differential input and output, comprising: a differentialinput stage configured to receive first and second differential inputsand to generate first and second positive currents and first and secondnegative currents therefrom; and an output stage configured to receiveand convert the first and second positive currents and the first andsecond negative currents into first and second differential outputvoltages on first and second outputs and to provide a feedback voltageto the differential input stage that is dependent on the output voltageson the first and second outputs, the output stage further comprising afirst resistive bridge comprising first and second resistors coupled inseries with each other and between the first output and a common modeterminal, and a second resistive bridge comprising third and fourthresistors coupled in series with each other and between the secondoutput and the common mode terminal, the output stage further comprisinga current-to-voltage conversion circuit having first and second inputterminals respectively provided for receiving a first current inputcomprising the sum of the first positive current and the second negativecurrent, and for receiving a second input current comprising the sum ofthe second positive current and the first negative current, andconfigured to generate a common mode voltage on the common modeterminal, and respectively generating first and second output voltageson the first and second outputs that are received by the first andsecond resistive bridges.
 9. A variable-gain amplifier with differentialinput and output, comprising: an attenuator block referenced to a commonmode voltage of an output of the amplifier, receiving an input voltageand adapted to providing, on a plurality of outputs, a plurality ofattenuated voltages, the attenuator block comprising two attenuatornetworks of R/2R type, respectively receiving first and second inputvoltages, each output of the attenuator block comprising an output nodeof each attenuator network, the output node generating an attenuatedvoltage equal to the input voltage received by the network andattenuated according to a predetermined ratio specific to the output; aplurality of differential transconductor elements controllable by acurrent, each differential transconductor element having a first inputconnected to an output of the attenuator block, each differentialtransconductor element generating first and second positive currents andfirst and second negative currents, a current source assembly adapted tocontrolling the transconductance of each differential transconductorelement according to an analog control signal, and an output blockconverting first and second input currents into a differential outputvoltage and providing a second input of each differential transconductorelement with a feedback voltage depending on the output voltage, thefirst input current being equal to the sum of the first positivecurrents and of the second negative currents and the second inputcurrent being equal to the sum of the second positive currents and ofthe first negative currents.
 10. A variable-gain amplifier withdifferential input and output, comprising: an attenuator blockreferenced to a common mode voltage of an output of the amplifier,receiving an input voltage and adapted to providing, on a plurality ofoutputs, a plurality of voltages, the attenuator block comprising twoattenuator networks of R/2R type, respectively receiving first andsecond input voltages, each output of the attenuator block comprising anoutput node of each attenuator network, the output node generating anattenuated voltage equal to the input voltage received by the networkand attenuated according to a predetermined ratio specific to theoutput; a plurality of differential transconductor elements controllableby a current, each differential transconductor element having a firstinput connected to an output of the attenuator block, each differentialtransconductor element generating first and second positive currents andfirst and second negative currents, each differential transconductorelement comprising two pairs of bipolar transistors, the emitters of thetransistors of a first pair being connected to a first current controlterminal of the differential transconductor element, and the emitters ofthe transistors of the second pair being connected to a second currentcontrol terminal of the differential transconductor element, the firstinput of the differential transconductor element being formed by thebases of the first two transistors of the two transistor pairs, thesecond input of the differential transconductor element being formed bythe bases of the two second transistors of the two transistor pairs, thetwo bases forming the first input of the differential transconductorelement being respectively connected to the output nodes of the outputof the attenuator block connected to the differential transconductorelement, the two bases forming the second input of the differentialtransconductor element being submitted to the feedback voltage, thecollectors of the first transistors generating the first and secondpositive currents, and the collectors of the second transistorsgenerating the first and second negative currents, a current sourceassembly adapted to controlling the transconductance of eachdifferential transconductor element according to an analog controlsignal, the current source assembly comprising pairs of controlterminals connected to the first and second current control terminals ofeach differential transconductor element, and an output block convertingfirst and second input currents into a differential output voltage andproviding a second input of each differential transconductor elementwith a feedback voltage depending on the output voltage, the first inputcurrent being equal to the sum of the first positive currents and of thesecond negative currents and the second input current being equal to thesum of the second positive currents and of the first negative currents,the output block comprising: a current-to-voltage conversion elementhaving first and second input terminals respectively provided forreceiving the first and second input currents, generating the commonmode voltage on a common mode output terminal, and respectivelygenerating first and second output voltages on first and second outputterminals, first and second resistive dividing bridges respectivelycoupled between the first and second output terminals of thecurrent-to-voltage conversion element and the common mode voltage, themidpoint of the first and second resistive dividing bridges generatingthe feedback voltage, and first and second current sources respectivelycoupled between a supply voltage and the first and second inputterminals of the output block.
 11. A variable-gain amplifier withdifferential input and output, comprising: an attenuator blockreferenced to a common mode voltage of an output of the amplifier,receiving an input voltage and adapted to providing, on a plurality ofoutputs, a plurality of voltages, the attenuator block comprising twoattenuator networks of R/2R type, respectively receiving first andsecond input voltages, each output of the attenuator block comprising anoutput node of each attenuator network, the output node generating anattenuated voltage equal to the input voltage received by the networkand attenuated according to a predetermined ratio specific to theoutput; differential transconductor elements controllable by a current,each differential transconductor element having a first input connectedto an output of the attenuator block, each differential transconductorelement generating first and second positive currents and first andsecond negative currents, each differential transconductor elementcomprising two pairs of bipolar transistors, the emitters of thetransistors of a first pair being connected to a first current controlterminal of the differential transconductor element, and the emitters ofthe transistors of the second pair being connected to a second currentcontrol terminal of the differential transconductor element, the firstinput of the differential transconductor element being formed by thebases of the first two transistors of the two transistor pairs, thesecond input of the differential transconductor element being formed bythe bases of the two second transistors of the two transistor pairs, thetwo bases forming the first input of the differential transconductorelement being respectively connected to the output nodes of the outputof the attenuator block connected to the differential transconductorelement, the two bases forming the second input of the differentialtransconductor element being submitted to the feedback voltage, thecollectors of the first transistors generating the first and secondpositive currents, and the collectors of the second transistorsgenerating the first and second negative currents, a current sourceassembly adapted to controlling the transconductance of eachdifferential transconductor element according to an analog controlsignal, the current source assembly comprising pairs of controlterminals connected to the first and second current control terminals ofeach differential transconductor element, and an output block convertingfirst and second input currents into a differential output voltage andproviding a second input of each differential transconductor elementwith a feedback voltage depending on the output voltage, the first inputcurrent being equal to the sum of the first positive currents and of thesecond negative currents and the second input current being equal to thesum of the second positive currents and of the first negative currents,the output block comprising: a current-to-voltage conversion elementhaving first and second input terminals respectively provided forreceiving the first and second input currents, generating the commonmode voltage on a common mode output terminal, and respectivelygenerating first and second output voltages on first and second outputterminals, the current-to-voltage conversion element comprising firstand second identical P-type MOS transistors, having their sourcesconnected to the supply voltage, having their gates connected to eachother, having their respective drains connected to the first and secondinput terminals of the current-to-voltage conversion element, third andfourth identical P-type MOS transistors, having their sources connectedto the supply voltage, having their respective gates connected to therespective drains of the first and second MOS transistors, having theirrespective drains connected to the first and second output terminals ofthe current-to-voltage conversion element, third and fourth currentsources arranged between the respective drains of the third and fourthtransistors and a first reference voltage, two identical resistors eachconnected between the respective drains of the third and fourthtransistors and the common mode output terminal, and a differentialamplifier having a first input connected to the common mode outputterminal, having a second input connected to a second reference voltage,and having its output connected to the gates of the first and secondtransistors; first and second resistive dividing bridges respectivelycoupled between the first and second output terminals of thecurrent-to-voltage conversion element and the common mode voltage, themidpoint of the first and second resistive dividing bridges generatingthe feedback voltage, and first and second current sources respectivelycoupled between a supply voltage and the first and second inputterminals of the output block.
 12. A variable-gain amplification methodfor use with a variable-gain amplifier having differential input anddifferential output, the method comprising: receiving an input voltagein an attenuator block and providing, on a plurality of outputs of theattenuator block, a plurality of voltages, each of the plurality ofvoltages equal to an attenuation of the input voltage in accordance witha predetermined attention ratio; receiving the attenuated input voltagesat respective differential transconductor elements and generatingtherefrom first and second positive currents and first and secondnegative currents; receiving an analog control signal at a currentsource assembly and controlling the transconductance of eachdifferential transconductor element responsive to the analog controlsignal; and receiving at an output block a first input currentcomprising the sum of the first positive current and the second negativecurrent, and further receiving a second input current that is the sum ofthe second positive current and the first negative current andconverting the same into differential output voltages and providing asecond input of each differential transconductor element with a feedbackvoltage that is dependent on the differential output voltages.
 13. Avariable-gain amplification method for use with a variable-gainamplifier having a differential input and differential output, themethod comprising: receiving an input voltage at an attenuator blockthat is referenced to a common mode voltage of an output of thevariable-gain amplifier, and generating on a plurality of outputs aplurality of voltages, each voltage equal to an attenuation of the inputvoltage according to a predetermined ratio; receiving from the pluralityof outputs the attenuated voltages that are controlled by a current,each differential transconductor element generating first and secondpositive currents and first and second negative currents; receiving ananalog control signal at a current source assembly and generating acurrent to control the transconductance of each differentialtransconductive element; and receiving at an output block a first inputcurrent comprising the sum of the first positive current and the secondnegative current and a second input current that is the sum of thesecond positive current and the first negative current and generating adifferential output voltage therefrom, and providing a second input ofeach differential transconductor element with a feedback voltage that isdependent on the differential output voltage.